Towards scalable, GCP-free UAV photogrammetry using PPP-RTK: repeatability tests at a Baltic Sea cliff site (Wustrow, Germany)

A major limitation of using UAV photogrammetry for coastal erosion and geohazard assessment is the effort associated with GCP-based georeferencing. To optimize this, we evaluate the German national PPP-RTK (precise point positioning real-time kinematic) service GEPOS®, provided by the Federal Agency for Cartography and Geodesy (BKG). The focus is on its ability to enable repeatable, GCP-free, and more time-efficient UAV-derived 3D products, suitable for operational upscaling.In total, we conducted 42 UAV surveys with three platforms (two DJI Mavic 3 Multispectral; RGB imagery only, and one DJI Mavic 3 Enterprise) in 10/2025, covering both nadir-only and multi-view oblique acquisition geometries at an actively eroding cliff section near Wustrow (Baltic Sea coast). Here a former East German military bunker collapsed in February 2024, thus providing a benchmark. GEPOS® corrections were generated by the BKG using three dedicated access points located within the area of interest and at distances of approximately 350 m and 1000 m. This setup enabled an initial sensitivity assessment of repeatability as a function of the correction-access configuration.In the absence of independent geodetic reference, we focused on repeatability (relative precision). To this end, we compared point clouds and DSMs across near-contemporaneous surveys to avoid impact of surface change. We applied a two-stage evaluation strategy: (i) we quantified end-to-end repeatability from unaligned model-to-model differences, reflecting combined georeferencing and photogrammetric effects; and (ii) removed rigid offsets by masked ICP-based co-registration on a stable concrete reference surface, transferring the derived rigid-body transform to the entire dataset before reassessing residual differences. We assessed distances using M3C2 and robust summary statistics such as Normalized Median Absolute Deviation (NMAD) and 95th and 99th percentiles of absolute M3C2 distances after excluding change-prone areas.Preliminary results indicate that oblique acquisition achieves centimeter-level repeatability without co-registration and improves to around the centimeter scale after co-registration. In contrast, nadir-only surveys show substantially larger inter-model discrepancies prior to co-registration, consistent with predominantly rigid offsets, but converge to low-centimeter residuals after alignment on stable surfaces. The final analysis will quantify repeatability across platforms, acquisition geometries, and correction-access configurations, and evaluate the implications for scaling UAV-based coastal monitoring while minimizing field effort.